Shawn,

I hope Kevin doesn't mind me jumping in here. I have a standard real-world example that I've used in the past to explain the concept of Interfaces. Kevin's example was good, but I thought you'd like another example also:

One of the biggest benefits of using Interfaces (in my opinion) is to be able to "program to the Interface", as they say. Let me explain with a real-world example:

Suppose I have created this Interface:

public interface IFillFromFinder
{
	void FillListView(DataSet dsList);
	void ClearListView();
}

Now, suppose I have a class that contains a ListView and implements the Interface:

public class MySearchClass : MyUserControl, IFillFromFinder
{
	#region Declarations

	protected ListView     oListView;
	protected MyFinderForm oFinder;

	#endregion

	#region Methods

	public void Search(ListView listView)
	{
		this.oListView = listview;
		this.ClearListView();

		this.oFinder = new MyFinderForm(this);
		oFinder.ShowDialog();
	}

	// Interface methods to implement 
	public void ClearListView()
	{
		// code here for clearing listview
	}

	public void FillListView(DataSet dsList)
	{
		// code here for filling listview
	}

	#endregion
}

As you can see, this class calls a Finder dialog Form, that gathers information from the User and performs a query against the backend data. If the Finder Dialog has been called from a control that implements this Interface (as in the above call from the Search() method), then it can also fill that control's ListView object by calling the methods on the Interface.

Here's the relevant part of my Finder Dialog. Note that the oListControl is defined using the Interface. MyFinderForm needs to know absolutely nothing else about what's calling it, other than that it implements the IFillFromFinder interface.

public class MyFinderForm : MyDialogForm	
{
	#region Declarations

	public  MyListDataSet   oData = new MyListDataSet();
	protected IFillFromFinder    oListControl = null;

	#endregion

	#region Constructors

	public MyFinderForm(IFillFromFinder CallingControl)
	{			
		this.oListControl = CallingControl;			
		InitializeComponent();
	}		

	public MyFinderForm()		
	{			
		InitializeComponent();
	}

	#endregion
		
	#region Events

	private void cmdOK_Click(object sender, System.EventArgs e)
	{
		this.FillDataSet();

		// If this dialog form was called by a class that implemented IFillFromFinder, 
		// then call call the Interface method to fill the ListView.
		if (this.oListControl != null)
		{
			if (this.chkReplace.Checked == true)
				this.oListControl.ClearListView();
			this.oListControl.FillListView(this.oData);
		}
	}


	#endregion
}

I hope this sort of helps explain interfaces by using a real world example.

~~Bonnie